Proprotein convertase subtilisin-kexin type 9 (hereinafter “PCSK9”), is a proteinase K-like subtilase identified as the 9th member of the mammalian PCSK family (Seidah et al, 2003 PNAS 100:928-933). The gene for PCSK9 localizes to human chromosome 1p33-p34.3, and is expressed in cells capable of proliferation and differentiation including, for example, hepatocytes, kidney mesenchymal cells, intestinal ileum, and colon epithelia as well as embryonic brain telencephalon neurons.
Structurally, PCSK9 consists of a 30 amino acid signal peptide, followed by a prodomain (amino acids 31-152), a catalytic domain (amino acids 153-425) and a CHRD (C-terminal cysteine/histidine-rich domain) (amino acids 426-692). PCSK9 is synthesized as an approximately 72 kDa precursor protein that undergoes autocatalytic cleavage in the endoplasmic reticulum. After cleavage, the approximately 14 kDa prodomain remains tightly associated with the active site, rendering the mature protein catalytically inactive. PCSK9 undergoes a variety of post-translational modifications before being secreted efficiently from the cell.
In cell-culture systems, overexpression of PCSK9 resulted in a decrease in LDL-receptor (LDLR) levels via a post-transcriptional mechanism (Park, et al, 2004 J. Biol. Chem. 279:50630-638). It has been shown that PCSK9 forms a direct protein-protein interaction with the EGF-A (epidermal growth factor-like repeat A) domain of the LDLR that results in targeting of the LDLR to lysosomes for degradation (Kwon et al, 2008 PNAS 105:1820-1825; Zhang, et al, 2007 J. Biol. Chem. 282:18602-18612). This interaction appears to be necessary for PCSK9-mediated LDLR degradation (Li et al, 2007 Biochem. J. 406:203-207; McNutt et al, 2007 J. Biol. Chem. 282:20799-20803). Studies using surface plasmon resonance demonstrated that direct binding of PCSK9 to the LDLR could be abolished with three different anti-PCSK9 antibodies, and these antibodies blocked the PCSK9-LDLR interaction by inhibiting the action of PCSK9 produced endogenously (Duff et al, 2009 Biochem. J. 419:577-584). A neutralizing anti-PCSK9 monoclonal antibody that binds an epitope on PCSK9 adjacent to the region required for LDLR interaction was shown to inhibit PCSK9 binding to the LDLR and attenuate PCSK9-mediated reduction in LDLR protein levels, thereby increasing LDL uptake. Additionally, a combination of this antibody with a statin increased LDLR levels in HepG2 cells more than either treatment alone (Chan et al, 2009 PNAS 106:9820-25). Subsequently, a fully human PCSK9 monoclonal antibody was shown to increase the recycling of LDL receptors and reduce LDL cholesterol levels (Roth et al, N Engl J Med 2012, 367:1891-900). Unfortunately, treatment to reduce LDL cholesterol by the administration of anti-PCSK9 antibodies is expensive and requires frequent administration of the antibodies to produce therapeutic effects.
Accordingly, it would be of great therapeutic benefit to produce a vaccine that could safely induce antibodies that antagonize the activity of PCSK9 for the treatment of various therapeutic conditions associated with PCSK9, such as increased plasma levels of LDL cholesterol.